BASIC AOTOMOTIVE (Gasoline Engine)

General Description
All gasoline engines mounted on Toyota automobiles are four-stroke engines.Four-stroke engines operate by continuous and regular repetition of the following sequence:
1. Intake stroke
2. Compression stroke3. Combustion stroke
4. Exhaust stroke
Basic principle of four-stroke engine
In order for an engine to operate smoothly under a wide range of operating conditions, the following three conditions must be satisfied:
1. Good air-fuel mixture
2. Good compression
3. Good spark

Good Air-fuel Mixture
1.Good air-fuel mixture for automobiles
In order for gasoline to completely burn, it must be sufficiently vaporized and mixed with air.
Automobiles are used under various operating conditions and there is a change in the engine operating condition, the required air-fuel mixture changes as well.When the air temperature changes from high to low.When the driving surface changes from flat ground to a steep incline and a heavy load is applied to the engine.When the engine speed changes widely from idling to high speed for acceleration.
2.Air-fuel ratio
The air-fuel ratio is the ratio of the mass of air to fuel.When the amount of air is too great or too small, the gasoline does not burn well, resulting in incomplete combustion.
There is a minimum of 14.7 parts air required to completely burn 1 part gasoline.
This is called the theoretical air-fuel ratio.However, in actual gasoline engines, even though the gasoline is injected to meet the theoretical air-fuel ratio, not all the gasoline can be vaporized and mixed with the air. For that reason, under some conditions a richer air-fuel mixture is necessary.
3.Air fuel ratio and driving conditions
At starting:
At starting, the walls of the intake manifold, the cylinders and the cylinder head are cold, causing the fuel sprayed by the injectors to adhere to them. In this case, the air-fuel mixture in the combustion chamber becomes lean. Therefore, the rich air-fuel mixture is required.
Warming-up:
The lower the coolant temperature is, the vaporized condition of the gasoline becomes worse, causing ignition worse. Therefore, the rich air-fuel mixture is required.
When accelerating:
When the accelerator pedal is depressed, a fuel supply lag occurs by load change, resulting in a leaner fuel mixture. Therefore, an additional amount of fuel is injected to the mixture.
When cruising (constant speed):
After the engine is completely warmed up, the fuel mixture supplied to the engine is very close to the theoretical air-fuel ratio.
Under heavy loads:
When a higher output of power is needed, a slightly richer fuel mixture is supplied to the engine to lower the burning temperature and ensure that all of the intake air is used in combustion.
When decelerating:
As engine output is not needed, fuel is cut-off in a part of this driving condition in order to clean exhaust gasses.
Good Compression
Necessity of compressing the air-fuel mixture
When an uncompressed air-fuel mixture is ignited, it burns slowly due to the low density of fuel and air. However, when compressed air-fuel mixture is ignited, the high density causes the mixture to suddenly burn (explode). Even when the fuel mixture is the same, a compressed mixture will release more power than an uncompressed mixture when ignited.Furthermore, compressing the air-fuel mixture causes the fuel and air to mix more thoroughly, resulting in a higher rate of vaporization of the gasoline and higher temperature when ignited. The compressed air-fuel mixture also burns more easily than before.The level of compression of the air-fuel mixture is expressed in the compression ratio. Generally, the greater explosive pressure can be attained when the compression pressure is higher. However the knocking occurs if the pressure is too high. Therefore, the compression ratio of the gasoline engine is normally designed to be between 9 to 11.
Good Spark
The gasoline engine converts the combustion of the air-fuel mixture to motive force.In order for the air-fuel mixture to burn well, it is important to have a powerful enough spark with proper ignition timing.
Ability to generate a powerful spark enough for the air-fuel mixture to burn (explode)
The spark plug in a gasoline engine generates a spark to burn the air-fuel mixture. If the spark is weak, there will not be enough energy to ignite the air-fuel mixture. For that reason, a powerful spark is essential
Ability to keep the proper ignition timing for every engine condition
The ignition timing changes according to engine speed or load to ensure that there is always proper ignition timing.
Description
The three essential elements of a gasoline engine are: good air-fuel mixture, good compression, and good spark.The ignition system generates a powerful spark through proper ignition timing in order to ignite the air-fuel mixture.
1.Powerful sparks
In the ignition system, sparks are generated between the electrodes of the spark plugs to burn the air-fuel mixture. Because even air has electrical resistance, when it is compressed highly, tens of thousands of volts must be generated to ensure the generation of powerful sparks that can ignite the air-fuel mixture.
2.Proper ignition timing
The ignition system must provide proper ignition timing at all times to accommodate the changes in engine speed and load.
3.Sufficient durability
The ignition system must be able to provide sufficient reliability to withstand the vibrations and heat that are generated by the engine.